Procedures to calculate the correlation function in the soft approximation
were presented in Refs. [1]}, [2]}, therefore we
skip further technical details and provide final expression for the
correlation function, which is calculated with dimension-9 accuracy and
given as a sum of the perturbative and nonperturbative components
\(\Pi ^{\mathrm {OPE}}(M^{2},s_{0}) &=&\frac{\mu _{K}}{8\pi ^{2}}\int _{\mathcal {M}^{2}}^{s_{0}}\frac{ds(m_{c}^{2}-s)^{2}}{s}e^{-s/M^{2}}+\Pi _{\mathrm {NP}}(M^{2}). \\&& \) 
